Call fasync() functions without the BKL

lock_kernel() calls have been pushed down into code which needs it, so
there is no need to take the BKL at this level anymore.

This work inspired and aided by Andi Kleen's unlocked_fasync() patches.

Acked-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>